Overlay PNG image as overlay in camera view in BGE
If I got it, you could parent a plane that uses the image as texture (textured using alpha), like this:



start enabling the "import images as planes" addon (bundled by default), and import your overlay image



then enable alpha for the texture if needed, and transparency for the material



then look at the camera rotation transforms 



and copy those values to the plane, and then move it towards the camera (translating on its local Z), to get:



in the end, here is an example of the camera rotating around a target object, and the view looking through the camera: